This invention relates to the measurement of physiological parameters associated with human speech, and is more particularly directed to electroglottograph apparatus, sometimes referred to as a laryngograph.
Because of the relative inaccessibility of the larynx or voice box, and the rapidity of the movements involved, it has been difficult to realize direct imaging of the movements of vibrating vocal folds of the human larynx. The imaging of the vocal fold movements during speech or singing generally requires techniques that are inconvenient and/or uncomfortable. For example, these direct imaging techniques can employ a mirror located in the oropharynx or a transnasal fiberoptic catheter. Other techniques, such as high-speed x-ray or stroboscopic photography, can present an unacceptable risk to the subject.
For the above reasons, a preferred technique for observing oscillatory movements of the vocal folds within the human larynx has been to use a small electric current through the neck at the level of the larynx to record the small changes in electrical impedance within the neck that are caused by the vocal fold movements. Preferably, an alternating current is applied for this, at a frequency of at least about one megahertz and a current of no more than about ten milliamperes.
An electroglottograph (EGG) is an electronic device for non-invasive measurement of the time variation of the transverse electrical impedance of the neck at the level of the larynx during voiced speech or singing, in order to obtain a waveform related to the vibratory patterns of the vocal folds within the larynx. In current practice, the electrode size and location, the nature of the electrical current, and other system parameters are chosen so as to make the voice-synchronous ac component of an electroglottograph output a waveform whose amplitude is related as closely as possible to the variation of vocal fold contact area (VFCA). It is this voice-synchronous ac component that is normally referred to as the EGG waveform. The variation in electrical impedance measured by the EGG waveform, and which is caused by the vocal fold contact area, is rarely much more than one or two percent of the total neck impedance, and in some subjects the variation can have a peak-to-peak value of less than 0.1% of the total neck impedance.
There are several commercially available electroglottograph devices, one of which is described in U.S. Pat. No. 4,139,732. That patent is addressed to the problem of efficiently demodulating an amplitude modulated carrier in which the modulation is only a small percentage of the carrier and proposes a technique in which only that part of the carrier waveform above a preset threshold level is demodulated, with feedback to the carrier source used to keep the amplitude of the carrier just above the threshold. The patent also addresses the problem of eliminating the electrical currents on the surface of the neck from the carrier to be demodulated, by means of a guard ring about each electrode.
The voice-synchronous component of the translaryngeal electrical impedance tends to repeat at the vibratory frequency of the vocal folds --a frequency which is generally above about 100 Hz and is rarely below about 50 Hz. However, an electroglottograph can also provide a lower frequency waveform that reflects to some degree the slower, non-vibratory motions within the larynx and movements of the entire larynx within the neck. Movements of this nature that are of greatest interest in the measurement of speech and singing include the gross abductory (pulling apart) or adductory (pressing together) movements of the vocal folds during phonation and vertical movements of the entire larynx structure. Such abductory/adductory movements and vertical movements occur continuously during natural speech or singing, both to satisfy the demands of vowel and consonant articulation and to enable the various parts of the larynx to adjust for the desired pitch and voice quality. A noninvasive system for measuring these parameters would be useful in various types of speech training, as, for example, foreign language instruction and speech training for the deaf. J. J. Mahshie, et al., Speech Training Aids for Hearing Impaired Individuals: III. Preliminary Observations in the Clinic and in Children's Homes, Journal of Rehabilitation Research and Development, Vol. 25, No. 4, pp. 69-82 (1988). Since vertical movements of the larynx are important in the mechanism of pitch control, they are also of great interest in singing pedagogy. All such non-vibratory movements are generally restricted to frequencies below about 20 Hz, since they are limited by the response times of the underlying skeletal musculature. Unfortunately, since the low frequency components of the translaryngeal impedance reflect many such effect simultaneously, there has been little success in measuring vocal fold abduction/adduction or vertical movements of the larynx using the low frequency EGG components. M. Rothenberg and J. J. Mashie, Monitoring Vocal Fold Abduction Through Vocal Fold Contact Area, J. Speech Hear Res., Vol. 31, pp. 338-351 (1988). Even for the measurement of VFCA, current techniques of electroglottography, while being potentially convenient, inexpensive, and non-invasive, still have residual problems. To obtain waveforms that represent primarily VFCA, current EGG units require careful and accurate placement of the electrodes with respect to the vocal folds. However, the larynx is often difficult to locate by conventional techniques such as palpation, and this is especially true with women and children and with subjects that are somewhat obese. Even with subjects for which the location of the larynx is easily observed, the continuous vertical migration of the larynx during speech or singing reduces the reliability of the output waveform. Moreover, with a small proportion of subjects, even an optimal positioning of the electrodes may not produce a waveform accurately depicting VFCA. Whether the problem in these cases is improper electrode position or neck physiology, the presently available units do not indicate unambiguously those occasions in which the signal provided by them is not to be trusted.
Yet another reason that electroglottography is not relied on by practitioners is that the various waveform features of most interest to the clinician have not yet been clearly charted. However, this is believed to result principally from the problems mentioned above, as it would constitute a waste of effort to document in detail artifacts, noise, and characteristics of a device that cannot be trusted. Finally, a limitation of all current techniques is that they indicate only the variation of the total contact area and do not differentiate in any way between different patterns of contact across the length of the vocal folds. D. G. Childers and A. K. Krishnamurthy, A Critical Review of Electroglottography, CRC Critical Reviews in Bioengineering, Vol.12, No. 2, pp. 131-161 (1985). Such differences in the contact pattern in the horizontal or anterior-posterior dimension are believed to be an important factor in determining vocal efficiency, and a convenient, noninvasive method for their measurement would be of significant value.